Scroll behavior correction

[Gazook89]

My last PR #2360 created a problem (i think, please double check if you can) where it doubled up the scrollbars on pages with iframes....so most of them.

This PR attempts to fix that error by narrowing the specificity of the rule.

There was also a overflow-y: overlay; leftover that I could remove as well.

Finally, it sets a scrollbar style on the Recent Items scrollbar. However, it only does so for firefox. I read up on the CSS for Chrome scrollbars and just couldn't get it work-- maybe someone else knows what is up:

.dropdown::-webkit-scrollbar-thumb {
  background : #333;

I can strip away the styling stuff and just leave the fix for the double scrollbar, too, if desired. In any case, this should finally close #1881 .

[G-Ambatte]

Looks like it works fine in both Windows Chrome & Firefox. The sort-container position is off-center in both browsers, but combined with the rework of that element in #2379, I think this is fine.

I also tested the Recently Viewed/Edited scrollbar, which also appears to work as intended.

[calculuschild]

Alright, I'm gonna merge this and take your word for it :+1: This is small enough I don't see it possibly breaking anything else.... 👀